Personally, I doubt either of those are correct! Scan some of the older threads for a fairly reliable method to calculate what you need. If you have your NG bills from the last year, and you can estimate the portion that was for heating verses cooking or other things (like maybe a dryer), and downloading the heating degree day info for your zipcode, you can get a very good indication of your real, measured, heat requirement.
My guess is that your need is closer to 30K BTU or so, but you'll never know until you do a good assessment. A properly sized boiler would run 100% on the coldest day of the year. Cycles cost money and shorten life. That's why matching the device to the load is so important, and it ends up being more comfortable, too.